Efficient Workload Balancing in Replicated Databases Based on Result Lag Computation

ABSTRACT

A computer system is configured to provide a database system. The computer system comprises one or more processors, a primary database system implemented by the one or more processors, and a secondary database system implemented by the one or more processors. The secondary database system is configured as a hot-standby system for the primary database system. The secondary database system is capable of providing at least a minimum amount of essential functionality of the primary database system during a disruption to the primary database system. The primary database system is configured by programming instructions, executable on the computer system, to cause the one or more processors to determine from a query request from a client application directed to the primary database system that workload from a query may be shifted to the secondary database system and selectively instruct the client application to direct the secondary database system to execute the query based on a per-table calculated result lag. Related apparatus, systems, techniques and articles are also described.

TECHNICAL FIELD

The subject matter described herein relates to database systems and moreparticularly to workload balancing between database systems employing aprimary database and a secondary database using enhanced techniques forcalculating per-table result lag.

BACKGROUND

A database system includes a database and a database management system(DBMS). A database is an organized collection of data. A DBMS comprisescomputer software that executes on one or more processors and interactswith users, other applications, and a database to capture and analyzedata. A DBMS may allow for the definition, creation, querying, update,and administration of databases.

SUMMARY

A computer system is configured to provide a database system. Thecomputer system comprises one or more processors, a primary databasesystem implemented by the one or more processors, and a secondarydatabase system implemented by the one or more processors. The secondarydatabase system is configured as a hot-standby system for the primarydatabase system. The secondary database system is capable of providingat least a minimum amount of essential functionality of the primarydatabase system during a disruption to the primary database system. Theprimary database system is configured by programming instructions,executable on the computer system, to cause the one or more processorsto determine from a query request from a client application directed tothe primary database system that workload from a query may be shifted tothe secondary database system and instruct the client application toselectively direct the secondary database system to execute the querybased on a computer per-table result lag.

These aspects and other embodiments may include one or more of thefollowing features. The programming instructions to cause the one ormore processors to determine from a query request from a clientapplication directed to the primary database system that workload from aquery may be shifted to the secondary database system comprisesprogramming instructions to cause the one or more processors to identifyall source tables implicated by the query having a corresponding replicatable and determine the calculated per-table result lag based on a lastcommit time for each source table and a last replayed transaction committimestamp for the corresponding replica table. With such variations, theper-table result lag can be a highest result lag value when there arenumerous source tables each having a replica.

The programming instructions configured to cause the one or moreprocessors to determine from a query request from a client applicationdirected to the primary database system that workload from a query maybe shifted to the secondary database system may comprise programminginstructions to cause the one or more processors to determine that arouting hint in the query request indicates that workload from the querymay be shifted to the secondary database system and determine that thequery does not involve writing data. The programming instructionsconfigured to cause the one or more processors to instruct the clientapplication to direct the secondary database system to execute the querymay comprise programming instructions to cause the one or moreprocessors to provide the identity of the secondary database system tothe client application.

The secondary database system may be configured by programminginstructions, executable on the computer system, to cause the one ormore processors to retrieve, responsive to receipt of the query from theclient application, a replication delay parameter from the query whereinthe replication delay parameter indicates the maximum acceptablereplication delay for data responsive to the query. The secondarydatabase system may further be configured by programming instructions,executable on the computer system, to retrieve the actual data lag fordata responsive to the query, compare the actual data lag with thereplication delay parameter, and provide the data responsive to query tothe client application when the actual data lag does not exceed thereplication delay parameter.

The secondary database system may be further configured by programminginstructions, executable on the computer system, to cause the one ormore processors to provide an indication to the client application toroute the query to the primary database system when the actual data lagexceeds the replication delay parameter. The secondary database systemmay be further configured by programming instructions, executable on thecomputer system, to cause the one or more processors to provide the dataresponsive to query and a fallback indication to the client applicationwhen the actual data lag exceeds the replication delay parameter. Thesecondary database system may be further configured by programminginstructions, executable on the computer system, to cause the one ormore processors to provide a fallback indication and not provide thedata responsive to the query to the client application when the actualdata lag exceeds the replication delay parameter.

In another embodiment, a computer-implemented method in a computersystem is provided. The computer system comprises a primary databasesystem and a secondary database system. The secondary database system isconfigured as a backup system for the primary database system and iscapable of providing at least a minimum amount of essentialfunctionality of the primary database system during a disruption to theprimary database system. The method comprises receiving by the primarydatabase system a query request from a client application in advance ofreceiving a query, determining by the primary database system that arouting hint in the query request indicates that workload from the querymay be shifted to the secondary database system, determining thatexecution of the query does not involve writing data, determining by theprimary database system to instruct the client application to route thequery to the secondary database system, and instructing, by the primarydatabase system, the client application to route the query to thesecondary database system.

These aspects and other embodiments may include one or more of thefollowing features. Instructing the client application to route thequery to the secondary database system may comprise providing theidentity of the secondary database system to the client application. Themethod may further comprise receiving the query from the clientapplication requesting data retrieval at the secondary database systemwhile in a hot-standby mode of operation. The method may furthercomprise retrieving, by the secondary database system, a replicationdelay parameter from the query wherein the replication delay parameterindicates the maximum acceptable replication delay for data responsiveto the query. The method may further comprise retrieving, by thesecondary database system, the actual data lag for the data responsiveto the query; comparing, by the secondary database system, the actualdata lag with the replication delay parameter; and providing, by thesecondary database system, the data responsive to the query when theactual data lag does not exceed the replication delay parameter. Themethod may further comprise providing an indication, by the secondarydatabase system, to the client application to route the query to theprimary database system when the actual data lag exceeds the replicationdelay parameter. The method may further comprise providing, by thesecondary database system, the data responsive to the query and afallback indication to the client application when the actual data lagexceeds the replication delay parameter. The method may further compriseproviding, by the secondary database system, a fallback indication tothe client application when the actual data lag exceeds the replicationdelay parameter.

In another embodiment, a non-transitory computer readable storage mediumembodying programming instruction for performing a method is provided.The method comprises receiving by the primary database system a queryrequest from a client application in advance of receiving a query,determining by the primary database system that a routing hint in thequery request indicates that workload from the query may be shifted tothe secondary database system wherein the secondary database systemconfigured as a backup system for the primary database system that iscapable of providing at least a minimum amount of essentialfunctionality of the primary database system during a disruption to theprimary database system, determining that execution of the query doesnot involve writing data, determining by the primary database system toinstruct the client application to route the query to the secondarydatabase system, and instructing, by the primary database system, theclient application to route the query to the secondary database system.

These aspects and other embodiments may include one or more of thefollowing features. Instructing the client application to route thequery to the secondary database system may comprise providing theidentity of the secondary database system to the client application. Themethod provided by the programming instructions embodied in thenon-transitory computer readable storage medium may further comprisereceiving the query from the client application requesting dataretrieval at the secondary database system while in a hot-standby modeof operation. The method may further comprise retrieving, by thesecondary database system, a replication delay parameter from the querywherein the replication delay parameter indicates the maximum acceptablereplication delay for data responsive to the query; retrieving, by thesecondary database system, the actual data lag for the data responsiveto the query; comparing, by the secondary database system, the actualdata lag with the replication delay parameter; and providing, by thesecondary database system, the data responsive to the query when theactual data lag does not exceed the replication delay parameter. Themethod may further comprise providing an indication, by the secondarydatabase system, to the client application to route the query to theprimary database system when the actual data lag exceeds the replicationdelay parameter. The method may further comprise providing, by thesecondary database system, the data responsive to the query and afallback indication to the client application when the actual data lagexceeds the replication delay parameter.

Non-transitory computer program products (i.e., physically embodiedcomputer program products) are also described that store instructions,which when executed by one or more data processors of one or morecomputing systems, cause at least one data processor to performoperations herein. Similarly, computer systems are also described thatmay include one or more data processors and memory coupled to the one ormore data processors. The memory may temporarily or permanently storeinstructions that cause at least one processor to perform one or more ofthe operations described herein. In addition, methods can be implementedby one or more data processors either within a single computing systemor distributed among two or more computing systems. Such computingsystems can be connected and can exchange data and/or commands or otherinstructions or the like via one or more connections, including but notlimited to a connection over a network (e.g., the Internet, a wirelesswide area network, a local area network, a wide area network, a wirednetwork, or the like), via a direct connection between one or more ofthe multiple computing systems, etc.

The subject matter described herein provides many technical advantages.As an example, the subject matter described herein may provide increasedaverage throughput for a database system during high workloads to reducethe likelihood that a request to the database system for data may bequeued, buffered or rejected until sufficient system resources areavailable to complete the request. In addition, the current subjectmatter provides more precise result lag by calculating result lag on aper-table basis which can avoid/help reduce unnecessary query fallbackto execute on source tables.

The details of one or more variations of the subject matter describedherein are set forth in the accompanying drawings and the descriptionbelow. Other features and advantages of the subject matter describedherein will be apparent from the description and drawings, and from theclaims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a system diagram illustrating an example database system foruse in connection with the current subject matter.

FIG. 2 is a system diagram illustrating an example database system thatcan support distribution of server components across multiple hosts forscalability and/or availability purposes for use in connection with thecurrent subject matter.

FIG. 3 is a diagram illustrating an architecture for an index server foruse in connection with the current subject matter.

FIG. 4 is a functional flow diagram illustrating an architecture tosupport load balancing between a primary database system and a secondarydatabase system, which serves as hot-standby to the primary databasesystem, for use in connection with the current subject matter.

FIG. 5 depicts one example solution to managing load balancing in aHA/DR system.

FIG. 6 depicts another example solution to managing load balancing in aHA/DR system.

FIG. 7 depicts an example architecture diagram for implementingHINT-based routing.

FIG. 8 depicts example operations of a HA/DR system when the data lagfor data responsive to a query exceeds a replication delay parameter.

FIG. 9 depicts an interface for defining a query having hint routing.

FIG. 10 depicts a flow diagram for an example process for more preciselycalculating result lag to allow for more efficient workloadbalancing/distribution.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

The subject matter described herein discloses apparatus, systems,techniques and articles that may provide increased average throughputcapabilities for a database system during high workloads to reduce thelikelihood that a request to the database system for data may be queued,buffered or rejected until sufficient system resources are available tocomplete the request. In some examples, apparatus, systems, techniquesand articles disclosed herein utilize secondary, backup database systemsto execute queries to reduce the workload of a primary database system.In some examples, systems and methods disclosed herein utilize a hint ina query request from an application to identify a query that may becandidate for execution by a secondary database system.

FIG. 1 is a diagram 100 illustrating a database system 105 that can beused to implement aspects of the current subject matter. The databasesystem 105 can, for example, be an in-memory database in which allrelevant data is kept in main memory so that read operations can beexecuted without disk I/O and in which disk storage is required to makeany changes durable. The database system 105 can include a plurality ofservers including, for example, one or more of an index server 110, aname server 115, and/or an application server 120. The database system105 can also include one or more of an extended store server 125, adatabase deployment infrastructure (DDI) server 130, a data provisioningserver 135, and/or a streaming cluster 140. The database system 105 canbe accessed by a plurality of remote clients 145, 150 via differentprotocols such as SQL/MDX (by way of the index server 110) and/orweb-based protocols such as HTTP (by way of the application server 120).

The index server 110 can contain in-memory data stores and engines forprocessing data. The index server 110 can also be accessed by remotetools (via, for example, SQL queries), that can provide variousdevelopment environment and administration tools. Additional detailsregarding an example implementation of the index server 110 is describedand illustrated in connection with diagram 300 of FIG. 3.

The name server 115 can own information about the topology of thedatabase system 105. In a distributed database system, the name server115 can know where various components are running and which data islocated on which server. In a database system 105 with multiple databasecontainers, the name server 115 can have information about existingdatabase containers, and it can also host the system database. Forexample, the name server 115 can manage the information about existingtenant databases. Unlike a name server 115 in a single-container system,the name server 115 in a database system 105 having multiple databasecontainers does not store topology information such as the location oftables in a distributed database. In a multi-container database system105 such database-level topology information can be stored as part ofthe catalogs of the tenant databases.

The application server 120 can enable native web applications used byone or more remote clients 150 accessing the database system 105 via aweb protocol such as HTTP. The application server 120 can allowdevelopers to write and run various database applications without theneed to run an additional application server. The application server 120can also be used to run web-based tools 155 for administration,life-cycle management and development. Other administration anddevelopment tools 160 can directly access the index server 110 for,example, via SQL and other protocols.

The extended store server 125 can be part of a dynamic tiering optionthat can include a high-performance disk-based column store for very bigdata up to the petabyte range and beyond. Less frequently accessed data(for which is it non-optimal to maintain in main memory of the indexserver 110) can be put into the extended store server 125. The dynamictiering of the extended store server 125 allows for hosting of verylarge databases with a reduced cost of ownership as compared toconventional arrangements.

The DDI server 130 can be a separate server process that is part of adatabase deployment infrastructure (DDI). The DDI can be a layer of thedatabase system 105 that simplifies the deployment of database objectsusing declarative design time artifacts. DDI can ensure a consistentdeployment, for example by guaranteeing that multiple objects aredeployed in the right sequence based on dependencies, and byimplementing a transactional all-or-nothing deployment.

The data provisioning server 135 can provide enterprise informationmanagement and enable capabilities such as data provisioning in realtime and batch mode, real-time data transformations, data qualityfunctions, adapters for various types of remote sources, and an adapterSDK for developing additional adapters.

The streaming cluster 140 allows for various types of data streams(i.e., data feeds, etc.) to be utilized by the database system 105. Thestreaming cluster 140 allows for both consumption of data streams andfor complex event processing.

FIG. 2 is a diagram 200 illustrating a variation of the database system105 that can support distribution of server components across multiplehosts for scalability and/or availability purposes. This database system105 can, for example, be identified by a single system ID (SID) and itis perceived as one unit from the perspective of an administrator, whocan install, update, start up, shut down, or backup the system as awhole. The different components of the database system 105 can share thesame metadata, and requests from client applications 230 can betransparently dispatched to different servers 110 ₁₋₃, 120 ₁₋₃, in thesystem, if required.

As is illustrated in FIG. 2, the distributed database system 105 can beinstalled on more than one host 210 ₁₋₃. Each host 210 ₁₋₃ is a machinethat can comprise at least one data processor (e.g., a CPU, etc.),memory, storage, a network interface, and an operation system and whichexecutes part of the database system 105. Each host 210 ₁₋₃ can executea database instance 220 ₁₋₃ which comprises the set of components of thedistributed database system 105 that are installed on one host 210 ₁₋₃.FIG. 2 shows a distributed system with three hosts, which each run aname server 110 ₁₋₃, index server 120 ₁₋₃, and so on (other componentsare omitted to simplify the illustration).

FIG. 3 is a diagram 300 illustrating an architecture for the indexserver 110 (which can, as indicated above, be one of many instances). Aconnection and session management component 302 can create and managesessions and connections for the client applications 145. For eachsession, a set of parameters can be maintained such as, for example,session variables, auto commit settings or the current transactionisolation level.

Requests from the client applications 145 can be processed and executedby way of a request processing and execution control component 310. Thedatabase system 105 offers rich programming capabilities for runningapplication-specific calculations inside the database system. Inaddition to SQL, MDX, and WIPE, the database system 105 can providedifferent programming languages for different use cases. SQLScript canbe used to write database procedures and user defined functions that canbe used in SQL statements. The L language is an imperative language,which can be used to implement operator logic that can be called bySQLScript procedures and for writing user-defined functions.

Once a session is established, client applications 145 typically use SQLstatements to communicate with the index server 110 which can be handledby a SQL processor 312 within the request processing and executioncontrol component 310. Analytical applications can use themultidimensional query language MDX (MultiDimensional eXpressions) viaan MDX processor 322. For graph data, applications can use GEM (GraphQuery and Manipulation) via a GEM processor 316, a graph query andmanipulation language. SQL statements and MDX queries can be sent overthe same connection with the client application 145 using the samenetwork communication protocol. GEM statements can be sent using abuilt-in SQL system procedure.

The index server 110 can include an authentication component 304 thatcan be invoked when a new connection with a client application 145 isestablished. Users can be authenticated either by the database system105 itself (login with user and password) or authentication can bedelegated to an external authentication provider. An authorizationmanager 306 can be invoked by other components of the database system105 to check whether the user has the required privileges to execute therequested operations.

Each statement can be processed in the context of a transaction. Newsessions can be implicitly assigned to a new transaction. The indexserver 110 can include a transaction manager 344 that coordinatestransactions, controls transactional isolation, and keeps track ofrunning and closed transactions. When a transaction is committed orrolled back, the transaction manager 344 can inform the involved enginesabout this event so they can execute necessary actions. The transactionmanager 344 can provide various types of concurrency control and it cancooperate with a persistence layer 346 to achieve atomic and durabletransactions.

Incoming SQL requests from the client applications 145 can be receivedby the SQL processor 312. Data manipulation statements can be executedby the SQL processor 312 itself. Other types of requests can bedelegated to the respective components. Data definition statements canbe dispatched to a metadata manager 308, transaction control statementscan be forwarded to the transaction manager 344, planning commands canbe routed to a planning engine 318, and task related commands canforwarded to a task manager 324 (which can be part of a larger taskframework) Incoming MDX requests can be delegated to the MDX processor322. Procedure calls can be forwarded to the procedure processor 314,which further dispatches the calls, for example to a calculation engine326, the GEM processor 316, a repository 330, or a DDI proxy 328.

The index server 110 can also include a planning engine 318 that allowsplanning applications, for instance for financial planning, to executebasic planning operations in the database layer. One such basicoperation is to create a new version of a data set as a copy of anexisting one while applying filters and transformations. For example,planning data for a new year can be created as a copy of the data fromthe previous year. Another example for a planning operation is thedisaggregation operation that distributes target values from higher tolower aggregation levels based on a distribution function.

The SQL processor 312 can include an enterprise performance management(EPM) runtime component 320 that can form part of a larger platformproviding an infrastructure for developing and running enterpriseperformance management applications on the database system 105. Whilethe planning engine 318 can provide basic planning operations, the EPMplatform provides a foundation for complete planning applications, basedon by application-specific planning models managed in the databasesystem 105.

The calculation engine 326 can provide a common infrastructure thatimplements various features such as SQLScript, MDX, GEM, tasks, andplanning operations. The SQL processor 312, the MDX processor 322, theplanning engine 318, the task manager 324, and the GEM processor 316 cantranslate the different programming languages, query languages, andmodels into a common representation that is optimized and executed bythe calculation engine 326. The calculation engine 326 can implementthose features using temporary results 340 which can be based, in part,on data within the relational stores 332.

Metadata can be accessed via the metadata manager component 308.Metadata, in this context, can comprise a variety of objects, such asdefinitions of relational tables, columns, views, indexes andprocedures. Metadata of all these types can be stored in one commondatabase catalog for all stores. The database catalog can be stored intables in a row store 336 forming part of a group of relational stores332. Other aspects of the database system 105 including, for example,support and multi-version concurrency control can also be used formetadata management. In distributed systems, central metadata is sharedacross servers and the metadata manager 308 can coordinate or otherwisemanage such sharing.

The relational stores 332 form the different data management componentsof the index server 110 and these relational stores can, for example,store data in main memory. The row store 336, a column store 338, and afederation component 334 are all relational data stores which canprovide access to data organized in relational tables. The column store338 can stores relational tables column-wise (i.e., in a column-orientedfashion, etc.). The column store 338 can also comprise text search andanalysis capabilities, support for spatial data, and operators andstorage for graph-structured data. With regard to graph-structured data,from an application viewpoint, the column store 338 could be viewed as anon-relational and schema-flexible in-memory data store forgraph-structured data. However, technically such a graph store is not aseparate physical data store. Instead it is built using the column store338, which can have a dedicated graph API.

The row store 336 can stores relational tables row-wise. When a table iscreated, the creator can specify whether it should be row orcolumn-based. Tables can be migrated between the two storage formats.While certain SQL extensions are only available for one kind of table(such as the “merge” command for column tables), standard SQL can beused on all tables. The index server 110 also provides functionality tocombine both kinds of tables in one statement (join, sub query, union).

The federation component 334 can be viewed as a virtual relational datastore. The federation component 334 can provide access to remote data inexternal data source system(s) 354 through virtual tables, which can beused in SQL queries in a fashion similar to normal tables.

The database system 105 can include an integration of a non-relationaldata store 342 into the index server 110. For example, thenon-relational data store 342 can have data represented as networks ofC++ objects, which can be persisted to disk. The non-relational datastore 342 can be used, for example, for optimization and planning tasksthat operate on large networks of data objects, for example in supplychain management. Unlike the row store 336 and the column store 338, thenon-relational data store 342 does not use relational tables; rather,objects can be directly stored in containers provided by the persistencelayer 346. Fixed size entry containers can be used to store objects ofone class. Persisted objects can be loaded via their persisted objectIDs, which can also be used to persist references between objects. Inaddition, access via in-memory indexes is supported. In that case, theobjects need to contain search keys. The in-memory search index iscreated on first access. The non-relational data store 342 can beintegrated with the transaction manager 344 to extends transactionmanagement with sub-transactions, and to also provide a differentlocking protocol and implementation of multi version concurrencycontrol.

An extended store is another relational store that can be used orotherwise form part of the database system 105. The extended store can,for example, be a disk-based column store optimized for managing verybig tables, which one may not want to keep in memory (as with therelational stores 332). The extended store can run in an extended storeserver 125 separate from the index server 110. The index server 110 canuse the federation component 334 to send SQL statements to the extendedstore server 125.

The persistence layer 346 is responsible for durability and atomicity oftransactions. The persistence layer 346 can ensure that the databasesystem 105 is restored to the most recent committed state after arestart and that transactions are either completely executed orcompletely undone. To achieve this goal in an efficient way, thepersistence layer 346 can use a combination of write-ahead logs, shadowpaging and savepoints. The persistence layer 346 can provide interfacesfor writing and reading persisted data and it can also contain a loggercomponent that manages a transaction log. Transaction log entries can bewritten explicitly by using a log interface or implicitly when using thevirtual file abstraction.

The persistence layer 236 stores data in persistent disk storage 348which, in turn, can include data volumes 350 and/or transaction logvolumes 352 that can be organized in pages. Different page sizes can besupported, for example, between 4k and 16M. Data can be loaded from thedisk storage 348 and stored to disk page wise. For read and writeaccess, pages can be loaded into a page buffer in memory. The pagebuffer need not have a minimum or maximum size, rather, all free memorynot used for other things can be used for the page buffer. If the memoryis needed elsewhere, least recently used pages can be removed from thecache. If a modified page is chosen to be removed, the page first needsto be persisted to disk storage 348. While the pages and the page bufferare managed by the persistence layer 346, the in-memory stores (i.e.,the relational stores 332) can access data within loaded pages.

In many applications, data systems may be required to support operationson a 24/7 schedule, and data system providers may be required toguarantee a maximum amount of downtime, that is time during which asystem is not able to fully support ongoing operations. When a system isrequired to ensure an agreed level of operational performance, it may bereferred to as a high availability system (“HA”). One solution toguarantee substantially continuous uptime with no, or very little,downtime is to maintain one or more hot-standby systems. A hot-standbysystem, or a backup system, is a system that may be activated quickly inthe event of a disruption causing one or more functions of a primaryoperational data system to fail. Such a disruption may be referred to asa disaster, and the process of restoring a data system to fulloperations may be referred to as disaster-recovery (“DR”).

A hot-standby system may be an exact replica of a primary operationalsystem that is capable of providing all the functions provided by theprimary operational system, or a hot-standby may be a system that iscapable of providing a minimum amount of essential functionality duringthe time required to restore the primary operational data system. Thetime it takes after a disaster to restore full, or minimum,functionality of a data system, for example by bringing a hot-standbyonline, is referred to as recovery time. In an effort to minimizerecovery time, and thereby downtime, a hot-standby system is typicallyin a state just short of fully operational. For example, a systemarchitecture may be implemented in which all functional systems of thehot-standby are active and operational, and all system and data changesor updates occur in the primary operational system and the hot-standbyat the exact same time. In such a case the only difference in the twosystems may be that the primary is configured to respond to userrequests and the hot-standby system is not. In other hot-standby systemsone or more functions may be disabled until mission critical systems ofthe hot-standby are observed to be operating normally, at which time theremaining functions may be brought online.

In many applications, data systems may be required to provide promptresponses to users and applications that rely on the data managed by thedata system. Providers and designers of data systems may be required toguarantee a minimum average throughput over time, or an average maximumresponse time. The speed with which a data system responds to a requestfrom a user or an application may be dependent on many factors, but allsystems are limited in the number of requests they can handle in a givenperiod of time. When a data system manages a relatively large amount ofdata, and supports a relatively large number of users or applications,during high workloads a request may be queued, buffered or rejecteduntil sufficient system resources are available to complete the request.When this happens, average throughput goes down and average responsetime goes up. One solution to such a problem is to distribute theworkload across multiple processing systems. This is known as loadbalancing.

One drawback to load balancing in HA systems is that load balancing mayrequire additional processing systems, which in turn have a high cost.It is often the case with certain data systems supporting criticalfunctions of an organization that additional systems are needed toperform both load balancing and HA functionality to efficiently supportcontinuous operations. Given the redundant nature of DR systems,hot-standby systems are often left undisturbed unless a disaster occurs.Thus, in some circumstances, it is desirable to implement and maintain acombination high availability/disaster recovery (HA/DR) system with loadbalancing that includes both a primary operational system and ahot-standby system, and potentially one or more tertiary systems. Such acombination system allows for load balancing of workload between theprocessing systems of both the primary operational system and thehot-standby system, without disrupting the ability of the hot-standbysystem to assume primary functionality in the event of a disaster.

FIG. 4 is a functional flow diagram illustrating an architecture 400 tosupport load balancing between a primary database system, or primarysystem 405 a and a secondary database system, or secondary system 405 b,which serves as hot-standby to primary system 405 a. Each of the primarysystem 405 a and the secondary system 405 b may be a single instancesystem, similar to database system 105 depicted in FIG. 1, or each maybe a distributed variation of database system 105 as depicted in FIG. 2.Such an architecture 400 may be useful in a high availability datasystem, or in a disaster recovery system, or in a combination HA/DRsystem.

Each of the primary system 405 a and secondary system 405 b may includea load balancing functionality. Such load balancing functionality mayfor example be contained within a distinct load balancing server 470 aor 470 b. But, such load balancing functionality may be managed by anysuitable processing system. For example, the application server 120 ofthe primary system may also manage the load balancing of requests issuedto the application server of the primary system 405 a, sending requeststo the secondary system 405 b as necessary to maintain a welldistributed workload.

As depicted in FIG. 4, each of the primary system 405 a and thesecondary system 405 b includes a load balancing server 470 a and 470 bwhich respectively receive requests from user applications directed tothe primary system 405 a or the secondary system 405 b. Such request maycome from either admin tools 460 or web-based tools 450, or any otheruser application. Upon receiving a request a load balancing server, e.g.470 a, determines how to distribute the workload. As depicted loadbalancing server 470 a routes an SQL request 465 from admin tools 460 tothe index server 110 of the primary system 405 a, while routing an HTTPrequest 455 from web-based tools 450 to the application server 120 ofthe secondary system 405 b.

Load balancing of resources between a primary system 405 a and asecondary system 405 b can give rise to a number of complicating issues.For example, if either of the requests 455, 465 requires writing to oneor more data tables, or modifying a data table, then the two systems 405a, 405 b will diverge. After many instances of write requests beingdistributed between the primary system 405 a and the secondary system405 b, the two systems would be substantially different, and likelyunusable. In another example, an application request, e.g. 465, mayperform a write transaction that is followed by a read transaction, e.g.455, related to the data written by the write request 465. If the writerequest is allocated to the primary system 405 a, the read request wouldobtain a different result depending on whether the subsequent readtransaction is carried out by the primary system 405 a or by thesecondary system 405 b.

Load balancing in a HA/DR system, by distributing a portion of theworkload of a primary data system to a hot-standby or backup system mustbe done in a way that does not disturb the principal purpose of thebackup system, which is to substantially eliminate downtime in a highavailability system by enabling quick and efficient recovery ofoperations. In other words, as a rule, load balancing cannot break thehot-standby. Given this principal purpose, any solution that enablesload balancing of workload between a primary system and a backup systemmust maintain the backup system in an identical, or nearly identical,state as the primary system. Such a solution should also avoid orprohibit any actions which may cause the state of the backup system tosubstantially diverge from the state of the primary system. In this way,in the event of a partial or total failure of the primary system due todisaster, the backup system can failover to a primary system mode withminimal or no impact to client applications.

FIG. 5 depicts one example solution to managing load balancing in aHA/DR system 500. HA/DR system 500 includes a primary system 505 and asecondary system 510 and is capable of load balancing between primarysystem 505 and secondary system 510 without interfering with thehot-standby functionality of the secondary system 510. Each of primarysystem 505 and secondary system 510 may be single instance databasesystems similar to database system 105 depicted in FIG. 1, or adistributed variation of database system 105 as depicted in FIG. 2.Furthermore, each of primary system 505 and secondary system 510 maycomprise less, more or all the functionality ascribed to index server110, 300, name server 115, application server 120, extended store server125, DDI server 130, data provisioning server 135, and stream cluster140. But, for simplicity of illustration HA/DR system 500 has beensimplified to highlight certain functionality by merely distinguishingbetween processing control 555, 560 and a persistence layer 565, 570 ofeach respective system 505, 510.

A collection of clients may each maintain an open connection to both theprimary system 505 and the secondary system 525. For example, client 515maintains a read/write connection 520 to the primary system 505 and aread only connection 525 to the secondary system 510. Alternatively,client 515 may maintain a read/write connection with each of the primarysystem 505 and the secondary system 510, while processes within thesecondary system 510 itself prohibit execution of any requests thatrequire a write transaction upon the secondary system while it is inbackup mode. Management of load balancing of the workload required by aclient application executing at client 515 may be managed by the client515 application itself. Alternatively, a client 515 application maysubmit a query request to the primary system 505. A process control 555load balancing process executing on processor 545 then may determinewhere the query should be executed and replies to the client 515 withinstructions identifying which system the client 515 should issue thequery to.

Primary system 505 may include an in-memory database in whichsubstantially all actively used data may be kept and maintained in mainmemory 535 so that operations can be executed without disk I/O, whichrequires accessing disk storage. Active operations of applicationswithin processing control 555 may cause processor 545 to read and writedata into main memory 535 or to disk in the persistence layer 565.Processing control 555 applications also cause processor 545 to generatetransaction logs for capturing data transactions upon the database,which processor 545 then persists in the log volumes 585. Assubstantially all actively used data may reside in-memory, processingcontrol 555 may interact primarily with data held in main memory whileonly resorting to data volumes 575 for retrieving and writing less oftenused data. Additional processes within processing control 555 may beexecuted by processor 545 to ensure that in-memory data is persisted inpersistence layer 565, so that the data is available upon restart orrecovery.

Primary system 505 may be the primary operational system for providingthe various functionality necessary to support 24/7 operations for anorganization. Secondary system 510 may be a hot-standby, ready to comeonline with minimal recovery time so as to minimize downtime. Secondarysystem 510 may be an identical physical system as primary system 505,and may be configured in a substantially identical manner in order toenable the secondary system 510 to provide all the same functionality asprimary system 505. For example, processing control 560 may include allthe same applications and functionality as processing control 555, andpersistence layer 570 may include data volumes 580 and log volumes 590that are configured in an identical manner as data volumes 575 and logvolumes 585 respectively. Secondary system 510 may also include anin-memory database kept and maintained primarily in main memory 540.

Primary system 505 and secondary system 510 differ in that all requests,from client 515 or otherwise, that require a write transaction areexecuted only in primary system 505. Primary system 505 and secondarysystem 510 further differ in that all write transactions to persistentobjects are prohibited by the secondary system 510. In order topropagate changes to the data or the underlying schema from the primarysystem 505 to the secondary system 510, processor 545 also replicates530 transaction logs directly to the process control 560 of thesecondary system 510. Process control 560 includes one or moreapplications that cause processor 550 to then replay the transactionlogs replicated from the primary system 505, thereby replaying thetransactions at the secondary system 510. As transaction logs arereplayed, the various transactions executed at the primary system becomereflected in the secondary system 510. In order to ensure both the HAfunctionality and the load balancing functionality, replay of thetransaction logs at the secondary system places data in main memory 540,and also persists any data committed in the primary system topersistence layer 570 to be stored by data volumes 580. Replay of thetransaction logs at the secondary system 510 may also results in thetransaction logs being persisted in log volumes 590.

Transaction logs may be replicated in different ways. Where maintaininga standby system in as close to the same state as the primary system isan important factor, logs may be replicated synchronously meaning thatthe primary system will not commit a transaction until the secondarysuccessfully responds to the log replication. One appreciates that thiswill slow performance of the primary system. Conversely, whereperformance of a primary system is a priority, logs may be replicatedasynchronously, in which case the primary operation proceeds withcommitting transactions without waiting for a response. Varioustradeoffs can be made between these two scenarios to achieve a properlevel of performance while ensuring replication of critical data.

It will be appreciated from the detailed description above that such asecondary system in standby mode, such as secondary system 510, can onlybe as current as its most recently replayed transaction logs.Transaction logs are replicated and replayed at the secondary system 510only after a transaction executes in the primary system 505. Secondarysystem 510, therefore, is always slightly behind an associated primarysystem 515. Also, there is no guarantee that a query routed to theprimary system in a load balancing effort will be executed before,during or after a particular transaction log is replayed. Thus, thestate of the primary system 505 and the state of the secondary systemwill rarely if ever be identical. But, by addressing certain concerns,secondary system 510 may be kept in a state substantially close to thesame state as the primary system 505 such that the workload required bymany operations can be supported by the secondary 510. These are just afew of the issues to be addressed in order to provide a robust loadbalancing implementation in a HA/DR architecture, where the hot-standbysystem also functions to carry a portion of the workload. One or moresolutions to issues arising by the load balancing solution depicted inFIG. 5 are now addressed.

FIG. 6 depicts another example solution to managing load balancing in aHA/DR system 600. HA/DR system 600 includes a primary database system605, a secondary database system 610, and a communication channel 655for propagating transaction logs and changes to data or the underlyingschema from the primary system 605 to the secondary system 610. TheHA/DR system 600 is capable of load balancing between primary system 605and secondary system 610 without interfering with the hot-standbyfunctionality of the secondary system 610. Each of primary system 605and secondary system 610 may be single instance database systems similarto database system 105 depicted in FIG. 1, or a distributed variation ofdatabase system 105 as depicted in FIG. 2. Furthermore, each of primarysystem 605 and secondary system 610 may comprise less, more or all thefunctionality ascribed to primary system 505 and secondary system 510,index server 110, 300, name server 115, application server 120, extendedstore server 125, DDI server 130, data provisioning server 135, andstream cluster 140. But, for simplicity of illustration HA/DR system 600has been simplified to highlight certain functionality by merelydistinguishing between index server 615, 620 and a session service 625,630 of each respective system 605, 610. Also, each of index server 615,620 may comprise less, more or all the functionality ascribed to indexserver 110, 300 and each of session service 625, 630 may comprise less,more or all the functionality ascribed to connection and sessionmanagement component 302.

A client application 634 may invoke one or more client libraries 635 toestablish connections with the primary and secondary server systems 605,610. As a result, one or more client libraries 635 may each maintain anopen connection to both the primary system 605 and the secondary system610. For example, client library 635 may maintain a read/writeconnection 645 to the primary system 605 and a read only connection 650to the secondary system 610. Alternatively, client library 635 maymaintain a read/write connection with each of the primary system 605 andthe secondary system 610, while processes within the secondary system610 itself prohibit execution of any requests that require a writetransaction upon the secondary system while it is in backup mode.

Management of load balancing of the workload required by a clientapplication 634, in this example, is managed by both the client library635 and the primary system 605. The client library 635 may submit aquery request to the session service 625 in the index server 615 of theprimary system 605. The client application 634 may indicate in the queryrequest through a hint that it is acceptable (or preferable) to theclient application 634 to have the query executed at the secondarysystem 610. A load balancing process executing on a processor within theindex server 615 in the primary system 605 then may determine where thequery should be executed and replies to the client 615 with instructionsidentifying the system to which the client 615 should issue the query.In this example, the index server 615 determines that the query shouldbe executed at the secondary system 610 and instructs the client library635 to issue the query to the secondary system 610. The hint mayindicate to the primary system 605 the preference of the client library635 for the query to execute at the secondary system if possible.

FIG. 7 depicts another example solution to managing load balancing in aHA/DR system 700. HA/DR system 700 includes a primary database system705 and a secondary database system 710 and is capable of load balancingbetween primary system 705 and secondary system 710 without interferingwith the hot-standby functionality of the secondary system 710. Each ofprimary system 705 and secondary system 710 may be single instancedatabase systems similar to database system 105 depicted in FIG. 1, or adistributed variation of database system 105 as depicted in FIG. 2.

Furthermore, each of primary system 705 and secondary system 710 maycomprise less, more or all the functionality ascribed to primary system605, 505 and secondary system 610, 510, index server 110, 300, nameserver 115, application server 120, extended store server 125, DDIserver 130, data provisioning server 135, and stream cluster 140. But,for simplicity of illustration HA/DR system 700 has been simplified tohighlight certain functionality by merely distinguishing between indexserver 715, 720 of each respective system 705, 710 and a session andconnection service 725, a SQL optimizer 765, and a name server 775 ofsystem 705. Also, each of index server 715, 720 may comprise less, moreor all the functionality ascribed to index server 110, 300, session andconnection service 725 may comprise less, more or all the functionalityascribed to connection and session management component 302, 625, SQLoptimizer 765 may comprise less, more or all the functionality ascribedto SQL processor 312, and name server 775 may comprise less, more or allthe functionality ascribed to name server 115.

A client application 734 may invoke one or more client libraries 735 toestablish connections with the primary and secondary server systems 705,710. As a result, one or more client libraries 735 may each maintain anopen connection to both the primary system 705 and the secondary system710. For example, client library 735 may maintain a read/writeconnection 745 to the primary system 705 and a read only connection 750to the secondary system 710. Alternatively, client library 735 maymaintain a read/write connection with each of the primary system 705 andthe secondary system 710, while processes within the secondary system710 itself prohibit execution of any requests that require a writetransaction upon the secondary system while it is in backup mode.

Management of load balancing of the workload required by a clientapplication 734, in this example, is managed by both the client library735 and the primary system 705. The client library 735 may submit aquery request to the session service 725 in the index server 715 of theprimary system 705. The client application 734 may indicate in the queryrequest through a hint that it is acceptable (or preferable) to theclient application 734 to have the query executed at the secondarysystem 710. A SQL optimizer 765 may parse the query request andrecognize the hint regarding query execution at the secondary system710. A load balancing process executing on a processor within the indexserver 715 in the primary system 705 then may determine where the queryshould be executed and replies to the client 715 with instructionsidentifying the system to which the client 715 should issue the query.

If it is determined that the query should be executed at the secondarysystem 710, the index server 715 may retrieve the secondary system'spublic name via a name server 775 and provide the secondary system'spublic name to the client library 735 in a reply. The client library 735may utilize a topology cache to store location information for data andmay need to have its topology cache extended to be made aware of thesecondary system 710.

In this example, the index server 715 determines that the query shouldbe executed at the secondary system 710 and instructs the client library735 to issue the query to the secondary system 710. The hint mayindicate to the primary system 705 the preference of the application 734for the query to execute at the secondary system if possible.

When executing a query at the secondary site, the secondary site couldreturn old data, for example, when the replay of delta logs is delayed.The client application 734 may specify a replication delay parameter(e.g., a time in seconds) as an acceptable lag time for the data whereinthe lag time is the difference between when a write transaction wascommitted on the primary system and when it has been replayed and can beread by the application on the secondary. The secondary system 710 mayprovide data to the client library 735 in response to a query only whena per-table result lag does not exceed the replication delay parameter.Alternatively, the secondary system 710 may instruct the client library735 to route the query to the primary database system when the per-tableresult lag exceeds the replication delay parameter. Or, the secondarysystem 710 may provide both the data responsive to the query and afallback indication to the client application when the per-table resultlag exceeds the replication delay parameter.

FIG. 8 depicts example operations of a HA/DR system 800 when the datalag for data responsive to a query exceeds a replication delayparameter. HA/DR system 800 includes a primary database system 805 and asecondary database system 810. Each of primary system 805 and secondarysystem 810 may be single instance database systems similar to databasesystem 105 depicted in FIG. 1, or a distributed variation of databasesystem 105 as depicted in FIG. 2. Furthermore, each of primary system805 and secondary system 810 may comprise less, more or all thefunctionality ascribed to primary system 705, 605, 505 and secondarysystem 710, 610, 510, index server 110, 300, name server 115,application server 120, extended store server 125, DDI server 130, dataprovisioning server 135, and stream cluster 140. But, for simplicity ofillustration HA/DR system 800 has been simplified to highlight certainfunctionality by merely distinguishing between index server 815, 820 ofeach respective system 805, 810 and a session and connection service825, a transaction manager or persistence layer 870, and a queryplanning engine 880. Also, each of index server 815, 820 may compriseless, more or all the functionality ascribed to index server 110, 300,615, 715; session and connection service 825 may comprise less, more orall the functionality ascribed to connection and session managementcomponent 302, 625,725; transaction manager or persistence layer 870 maycomprise less, more or all the functionality ascribed to transactionmanager 344 and/or persistence layer 345, and query planning engine 880may comprise less, more or all the functionality ascribed to queryplanning engine 318.

After receiving instructions to route a query to the secondary system810, the client library 835, invoked by the client application 834,instructs the secondary system 810 to execute a routed query statement(operation 802). The query planning engine 880 retrieves the specifiedlag (i.e., replication delay parameter) from the query (operation 804).The transaction manager or persistence layer 870 retrieves the currentdata lag for the data responsive to the query (operation 806). Thesession and connection service 825 compares the specified lag to thecurrent data lag and decides on whether data responsive to the queryshould be provided to the client library 835 (operation 808). When thecurrent data lag exceeds the specified lag, the secondary system willnotify the client library (operation 812). Upon receiving notificationthat the current data lag exceeds the specified lag, the client library835, reroutes the query to the primary system 805 for execution(operation 814).

The primary system 605 and the secondary system 610 can be configured toprovide a replication option referred to as Asynchronous TableReplication (ATR), which focuses on load balancing and scalable readperformance by replicating a selected list of tables within a singledata center.

A query with an ATR hint will be executed on a replica table if theactually computed result lag (i.e. current replay delay of the replicanode) is no greater than the one specified by user. For example, seediagram 900 of FIG. 9 and the following:

 --Create table T1 on master indexserver  create column table T1(a int,b int) at ‘lint12rt:35003’;  --A replica table of T1 named _SYS_REP_T1#0is created on a slave indexserver, T1 is called source table and_SYS_REP_T1#0 is called replica table  alter table T1 add asynchronousreplica at ‘lint12rt:35040’;  --Enable replica table _SYS_REP_T1#0 ALTER TABLE T1 ENABLE ASYNCHRONOUS REPLICA;  --Define a query hintusing ATR  ALTER SYSTEM ALTER CONFIGURATION (‘indexserver.ini’,‘system’) SET (‘hint_result_lag_hana_atr’, ‘enable_features’) = ‘atr’WITH RECONFIGURE;  --Execute a query with ATR hint, if the actuallycomputed  result lag is no greater than 20 seconds, then the query isexecuted on _SYS_REP_T1#0 instead of source table T1, which is shown inFIG. 1  SELECT * FROM T1 WITH HINT  (RESULT_LAG(‘hana_atr’, 20));

As mentioned above, the query will be executed on replica table if theactually computed result lag is no greater than the one specified byuser, otherwise it will fall back to recompile and execute on sourcetable. The actual result lag calculation in some variations is calledper-node result lag computation, i.e. it is calculated during queryexecution by subtracting the minimum source commit time of relatedindexservers where the query involved tables each having a replicareside from current time.

That is to say, currently result lag is calculated per-node(indexserver) level, which is not accurate, e.g. assuming tables TABLE1with last commit time t1 and TABLE2 with last commit time t2, wheret1>t2; both tables reside on indexserver 1, and each has a replica named_SYS_REP_TABLE1#0 and _SYS_REP_TABLE2#0 respectively residing onindexserver 2.

Consider the query below:

SELECT * FROM TABLE1 WITH HINT(RESULT LAG(‘hana_atr’, 20))

According to some approaches, result lag=current time−last commit timet2 of table T2 (the minimum commit time on indexserver 1). With thisarrangement, a greater result lag is gained, which might result in thatreplica _SYS_REP_TABLE1#0 being given up upon query execution which, inturn, causes the query to unnecessarily fall back to execute on sourcetable TABLE1. This occurs despite TABLE2 not being related to the query.

FIG. 10 is a process flow diagram 1000 which illustrates an arrangementfor per-table result lag computation, which is more accurate, i.e.calculating result lag based on involved tables of the query, not basedon related indexservers where the query involved tables each having areplica reside. A query is received from a client application and, at1010, the query is compiled such that the replicable tables replacessource tables. Based on the compiled query, at 1020, a query executionplan is generated which specifies various aspects on how the query willbe executed. Thereafter, at 1030, a result lag is calculated on aper-table basis. In some variations, the result lag is calculated bysubtracting a minimum commit time of related index servers having tablesimplicated by the query have a corresponding replica. If the calculatedresult flag is lower than a pre-defined threshold (e.g., a userspecified threshold), then the query can, at 1060, be directly executedusing the query execution plan. Otherwise, if the result lag is abovethe pre-defined threshold, then at 1050, the query falls back from thereplica table to the source table (due to the replica table not beingavailable, etc.) causing the query to get compiled again.

The per-table result lag computation can be as follows:

(1) Mantain a map: SourceCommitTimeMap<SourceTableOid, LastCommitTime>by periodically (each 10 seconds) getting the last commit time for eachsource table having a replica;

(2) Maintain the last replayed transaction commit timestamp for eachreplica table, call it LastReplayedTxnCommitTime;

(3) For each source table, say table Tab1 here for example:

if (SourceCommitTimeMap.probe(Tab1_OID) < LastReplayedTxnCommitTime)  //This means that the all transactions involving this table have beenreplayed. Hence, the conservative result lag should be only as much asthe time which has passed since the last time the timer thread (whichtriggers every 10 secs) was invoked  ResultLag_Tab1 =(CurrentWallClockTime − LastTimerThreadCollectionWallClockTime ); else //This means that there are some commits on this table beyond what wehave replayed in the ATR replayer  ResultLag_Tabl = (CurrentWallClockTime − LastTimerThreadCollectionWallClockTime) +(SourceCommitTimeMap.probe(Tabl_OID) − LastReplayedTxnCommitTime);

(4) For a query involving multiple source tables each having a replica,the actual result lag is the maximum of result lags computed across allreplicas.

One or more aspects or features of the subject matter described hereincan be realized in digital electronic circuitry, integrated circuitry,specially designed application specific integrated circuits (ASICs),field programmable gate arrays (FPGAs) computer hardware, firmware,software, and/or combinations thereof. These various aspects or featurescan include implementation in one or more computer programs that areexecutable and/or interpretable on a programmable system including atleast one programmable processor, which can be special or generalpurpose, coupled to receive data and instructions from, and to transmitdata and instructions to, a storage system, at least one input device,and at least one output device. The programmable system or computingsystem may include clients and servers. A client and server aregenerally remote from each other and typically interact through acommunication network. The relationship of client and server arises byvirtue of computer programs running on the respective computers andhaving a client-server relationship to each other.

These computer programs, which can also be referred to as programs,software, software applications, applications, components, or code,include machine instructions for a programmable processor, and can beimplemented in a high-level procedural language, an object-orientedprogramming language, a functional programming language, a logicalprogramming language, and/or in assembly/machine language. As usedherein, the term “machine-readable medium” refers to any computerprogram product, apparatus and/or device, such as for example magneticdiscs, optical disks, memory, and Programmable Logic Devices (PLDs),used to provide machine instructions and/or data to a programmableprocessor, including a machine-readable medium that receives machineinstructions as a machine-readable signal. The term “machine-readablesignal” refers to any signal used to provide machine instructions and/ordata to a programmable processor. The machine-readable medium can storesuch machine instructions non-transitorily, such as for example as woulda non-transient solid-state memory or a magnetic hard drive or anyequivalent storage medium. The machine-readable medium can alternativelyor additionally store such machine instructions in a transient manner,such as for example as would a processor cache or other random accessmemory associated with one or more physical processor cores.

To provide for interaction with a user, the subject matter describedherein may be implemented on a computer having a display device (e.g., aCRT (cathode ray tube) or LCD (liquid crystal display) monitor) fordisplaying information to the user and a keyboard and a pointing device(e.g., a mouse or a trackball) and/or a touch screen by which the usermay provide input to the computer. Other kinds of devices may be used toprovide for interaction with a user as well; for example, feedbackprovided to the user may be any form of sensory feedback (e.g., visualfeedback, auditory feedback, or tactile feedback); and input from theuser may be received in any form, including acoustic, speech, or tactileinput.

In the descriptions above and in the claims, phrases such as “at leastone of” or “one or more of” may occur followed by a conjunctive list ofelements or features. The term “and/or” may also occur in a list of twoor more elements or features. Unless otherwise implicitly or explicitlycontradicted by the context in which it is used, such a phrase isintended to mean any of the listed elements or features individually orany of the recited elements or features in combination with any of theother recited elements or features. For example, the phrases “at leastone of A and B;” “one or more of A and B;” and “A and/or B” are eachintended to mean “A alone, B alone, or A and B together.” A similarinterpretation is also intended for lists including three or more items.For example, the phrases “at least one of A, B, and C;” “one or more ofA, B, and C;” and “A, B, and/or C” are each intended to mean “A alone, Balone, C alone, A and B together, A and C together, B and C together, orA and B and C together.” In addition, use of the term “based on,” aboveand in the claims is intended to mean, “based at least in part on,” suchthat an unrecited feature or element is also permissible.

The subject matter described herein can be embodied in systems,apparatus, methods, and/or articles depending on the desiredconfiguration. The implementations set forth in the foregoingdescription do not represent all implementations consistent with thesubject matter described herein. Instead, they are merely some examplesconsistent with aspects related to the described subject matter.Although a few variations have been described in detail above, othermodifications or additions are possible. In particular, further featuresand/or variations can be provided in addition to those set forth herein.For example, the implementations described above can be directed tovarious combinations and subcombinations of the disclosed featuresand/or combinations and subcombinations of several further featuresdisclosed above. In addition, the logic flows depicted in theaccompanying figures and/or described herein do not necessarily requirethe particular order shown, or sequential order, to achieve desirableresults. Other implementations may be within the scope of the followingclaims.

What is claimed is:
 1. A computer system comprising: one or moreprocessors; a primary database system implemented by the one or moreprocessors; and a secondary database system implemented by the one ormore processors, the secondary database system configured as ahot-standby system for the primary database system that is capable ofproviding at least a minimum amount of essential functionality of theprimary database system during a disruption to the primary databasesystem; wherein the primary database system is configured by programminginstructions, executable on the computer system, to cause the one ormore processors to: determine from a query request from a clientapplication directed to the primary database system that workload from aquery may be shifted to the secondary database system, and selectivelyinstruct the client application to direct the secondary database systemto execute the query based on a calculated per-table result lag.
 2. Thecomputer system according to claim 1, wherein the programminginstructions to cause the one or more processors to determine from thequery request from the client application directed to the primarydatabase system that workload from a query may be shifted to thesecondary database system comprises programming instructions to causethe one or more processors to: identify all source tables implicated bythe query having a corresponding replica table; and determine thecalculated per-table result lag based on a last commit time for eachsource table and a last replayed transaction commit timestamp for thecorresponding replica table.
 3. The computer system according to claim2, wherein the per-table result lag is a highest result lag value whenthere are numerous source tables each having a replica.
 4. The computersystem according to claim 1, wherein the programming instructions tocause the one or more processors to determine from a query request froma client application directed to the primary database system thatworkload from a query may be shifted to the secondary database systemcomprises programming instructions to cause the one or more processorsto: determine that a routing hint in the query request indicates thatworkload from the query may be shifted to the secondary database system;and determine that the query does not involve writing data.
 5. Thecomputer system according to claim 1, wherein the programminginstructions to cause the one or more processors to instruct the clientapplication to direct the secondary database system to execute the querycomprises programming instructions to cause the one or more processorsto provide the identity of the second database system to the clientapplication.
 6. The computer system according to claim 1, wherein thesecondary database system is configured by programming instructions,executable on the computer system, to cause the one or more processorsto: retrieve, responsive to receipt of the query from the clientapplication, a replication delay parameter from the query, thereplication delay parameter indicating the maximum acceptablereplication delay for data responsive to the query, calculate theper-table result lag for data responsive to the query, compare theper-table result lag with the replication delay parameter, provide thedata responsive to query to the client application when the per-tableresult lag does not exceed the replication delay parameter.
 7. Thecomputer system according to claim 6, wherein the secondary databasesystem is further configured by programming instructions, executable onthe computer system, to cause the one or more processors to: provide anindication to the client application to route the query to the primarydatabase system when the per-table result lag exceeds the replicationdelay parameter.
 8. The computer system according to claim 6, whereinthe secondary database system is further configured by programminginstructions, executable on the computer system, to cause the one ormore processors to: provide the data responsive to query and a fallbackindication to the client application when the per-table result lagexceeds the replication delay parameter.
 9. The computer systemaccording to claim 6, wherein the secondary database system is furtherconfigured by programming instructions, executable on the computersystem, to cause the one or more processors to: provide a fallbackindication and not provide the data responsive to the query to theclient application when the per-table result lag exceeds the replicationdelay parameter.
 10. A computer-implemented method in a computer systemcomprising a primary database system and a secondary database system,the secondary database system configured as a backup system for theprimary database system that is capable of providing at least a minimumamount of essential functionality of the primary database system duringa disruption to the primary database system, the method comprising:receiving by the primary database system a query request from a clientapplication in advance of receiving a query, determining by the primarydatabase system that a routing hint in the query request indicates thatworkload from the query may be shifted to the secondary database system;determining that execution of the query does not involve writing data;determining by the primary database system to instruct the clientapplication to route the query to the secondary database system; andselectively instructing, by the primary database system, the clientapplication to route the query to the secondary database system based ona calculated per-table result lag.
 11. The method according to claim 10,further comprising: identifying all source tables implicated by thequery having a corresponding replica table; and determining thecalculated per-table result lag based on a last commit time for eachsource table and a last replayed transaction commit timestamp for thecorresponding replica table.
 12. The method according to claim 11,wherein the per-table result lag is a highest result lag value whenthere are numerous source tables each having a replica.
 13. The methodaccording to claim 8, wherein instructing the client application toroute the query to the secondary database system comprises providing theidentity of the secondary database system to the client application. 14.The method according to claim 10, further comprising: receiving thequery from the client application requesting data retrieval at thesecondary database system while in a hot-standby mode of operation. 15.The method according to claim 12, further comprising: retrieving, by thesecondary database system, a replication delay parameter from the query,the replication delay parameter indicating the maximum acceptablereplication delay for data responsive to the query; retrieving, by thesecondary database system, the per-table result lag for the dataresponsive to the query; comparing, by the secondary database system,the per-table result lag with the replication delay parameter; andproviding, by the secondary database system, the data responsive to thequery when the per-table result lag does not exceed the replicationdelay parameter.
 16. The method according to claim 15, furthercomprising: providing an indication, by the secondary database system,to the client application to route the query to the primary databasesystem when the per-table result lag exceeds the replication delayparameter.
 17. The method according to claim 15, further comprising:providing, by the secondary database system, the data responsive to thequery and a fallback indication to the client application when theper-table result lag exceeds the replication delay parameter.
 17. Themethod according to claim 15, further comprising: providing, by thesecondary database system, a fallback indication to the clientapplication when the per-table result lag exceeds the replication delayparameter.
 18. A non-transitory computer readable storage mediumembodying programming instruction for performing a method, the methodcomprising: receiving by a primary database system a query request froma client application in advance of receiving a query, determining by theprimary database system that a routing hint in the query requestindicates that workload from the query may be shifted to a secondarydatabase system, the secondary database system configured as a backupsystem for the primary database system that is capable of providing atleast a minimum amount of essential functionality of the primarydatabase system during a disruption to the primary database system;determining that execution of the query does not involve writing data;determining by the primary database system to instruct the clientapplication to route the query to the secondary database system; andselectively instructing, by the primary database system, the clientapplication to route the query to the secondary database system based ona calculated per-table result lag.
 19. The non-transitory computerreadable storage medium according to claim 18, wherein the methodfurther comprises: identifying all source tables implicated by the queryhaving a corresponding replica table; and determining the calculatedper-table result lag based on a last commit time for each source tableand a last replayed transaction commit timestamp for the correspondingreplica table.
 20. The non-transitory computer readable storage mediumaccording to claim 19, wherein the method further comprises, wherein theper-table result lag is a highest result lag value when there arenumerous source tables each having a replica. providing an indication,by the secondary database system, to the client application to route thequery to the primary database system when the per-table result lagexceeds the replication delay parameter.